Printers are useful for producing printed images of a wide range of types. Printers print on receivers (or “imaging substrates”), such as pieces or sheets of paper or other planar media, glass, fabric, metal, or other objects. Printers typically operate using subtractive color: a substantially reflective receiver is overcoated image-wise with cyan (C), magenta (M), yellow (Y), black (K), and other colorants. Various schemes can be used to process images to be printed.
This application is related to commonly assigned, co-pending U.S. Patent Publication No. 2010/0097657, filed Oct. 12, 2009, entitled “Adaptive Exposure Printing and Printing System,” by Kuo et al., the disclosure of which is incorporated herein by reference.
Electrophotographic (EP) printers have an optical writer that exposes a row at a time in the cross-track direction. The receiver is sequenced through the printer in the in-track direction, which is substantially perpendicular to the cross-track direction. However, printers can have nonuniformities. For example, non-uniform exposure of an area intended to be constant density on the receiver can result in a streak, an area unintentionally exposed differently than its surround, extending in the slow-scan direction. “Streaks” are columnar visible artifacts produced by differences between one pixel and the next in the cross-track direction. “Bands” are row-oriented visible artifacts produced by variations over time as the receiver moves through the printer. Various schemes have been proposed for correcting streaks and bands. U.S. Pat. No. 7,058,325 to Hamby et al. deposits a test patch, measures its density, and corrects using a feedback or feedforward control routine. U.S. Patent Publication No. 20080226361 by Tomita et al. describes measuring multiple patterns, each containing multiple rows of toner, possibly set at different angles on the page, and combining the measurement results to determine image adjustments. U.S. Pat. No. 7,564,475 to Mizes prints a test pattern and senses its reflectance (density), then determines frequency, amplitude, or phase of banding based on the sensed test pattern. The intensity of a laser beam is adjusted to compensate for the banding. U.S. Pat. No. 7,663,654 to Arai et al. adjusts light-emission period of LEDs to correct for periodic variations.
U.S. Pat. No. 6,452,696 to Bogart et al. describes normalizing the output of multiple light sources. Each light source is driven by an individual digital-to-analog converter (DAC). However, the requirement for a DAC for each light source can become prohibitive as resolution, and therefore the number of light sources, increases. U.S. Pat. No. 6,554,388 to Wong et al. describes computing an exposure element average density value using measurements of a test patch. A non-uniformity correction value is computed based on a difference between the exposure element average density value and an expected average.
To accommodate hardware limitations and reduce noise, EP printers typically use screened patterns (e.g. halftones) rather than continuous tones. Marks on the receiver are placed according to a variety of geometrical patterns so that a group of marks, when seen by the eye, gives a rendition of a desired intermediate color tone between the color of the background (e.g. paper stock) and the color of the mark. U.S. Pat. No. 5,485,289 to Curry, and commonly assigned EP 0 892 549 B1 to Tai et al., describe various methods for halftoning and designing screening patterns.
Tai et al. also recognize non-uniformities in EP printheads that can produce streaks. Some streaks are consistently lighter or darker than their surrounds, corresponding e.g. to consistent over- or under-exposure on the photoreceptor. However, some streaks are lighter than their surround in some areas and darker than their surround in other areas. There is a need to compensate for both types of streaks.
U.S. Patent Application Publication No. 2005/0036705 to Viassolo et al., and U.S. Pat. No. 7,038,816 to Klassen et al., describe systems to reduce streaking by adjusting tone reproduction curve (TRC) values. However, adjusting TRC values confounds streaking-reduction with the intended purpose of TRC values, which is compensating for device non-linearities. This can increase memory requirements of a printer and restrict the available compensation to the range of adjustment provided by the TRC.
U.S. Patent Application Publication No. 2005/0134624 to Mizes describes various test patterns that can be printed on a receiver and scanned to determine streaking-compensation values. “Scanner-based technique to adjust LED printbar uniformity” by Mizes et al. (IS&T NIP19 pp. 532-536, ISBN 0-89208-247-X, dated Sep. 28, 2003) also describes test patterns and schemes for compensation. “Automatic density control for increased print uniformity and printer reliability with inline linear array sensing” by Mizes et al. (IS&T NIP24 pp. 206-210, ISBN 978-0-89208-279-7, dated Sep. 6, 2008) describes capturing an image of a test pattern strip to perform compensation. However, these schemes use TRCs for compensation, so suffer from the same limitations as Viassolo et al. and Klassen et al. Additionally, Mizes et al. (NIP19) disclose that some observers can perceive density variations with a peak-to-peak amplitude of only 0.25 ΔL*. However, Mizes et al. require extensive and time-consuming measurements to reach high precision.
There is a continuing need, therefore, for an improved way of compensating for streaks and other non-uniformities in a hardcopy reproduction apparatus.